The present invention relates to inductive antenna circuits, particularly for contactless integrated circuit readers, and more particularly, to an antenna circuit tunable to a determined resonance frequency, comprising an antenna coil and adjustable components. The present invention further relates to a method for tuning such an antenna circuit.
FIG. 1 schematically represents a contactless integrated circuit reader RD equipped with an antenna circuit 10 of the aforementioned type. The antenna circuit 10 comprises a coil L and adjustable components, such as two adjustable capacitors C1, C2. The antenna circuit receives an alternating current (AC) excitation signal Se of frequency F0 supplied by the reader RD, and sends an AC magnetic field FLD. This magnetic field enables the reader RD to communicate by inductive coupling with contactless integrated circuits CIC1, CIC2 . . . CICn which are each equipped with a respective antenna coil L1, L2 , . . . Ln. The integrated circuits CIC1–CICn are, for example, contactless smart card integrated circuits, contactless electronic tag integrated circuits, contactless badge integrated circuits, and the like.
The adjustable capacitors C1, C2 enable the resonance frequency of the antenna circuit to be tuned to the frequency F0 of the excitation signal, which is for example 13.56 Mega Hertz (MHz) according to the International Standards Organization (ISO) standards ISO 15693 and ISO 14443. The tuning of the antenna circuit to the frequency F0 enables an optimal communication distance to be obtained between the reader and the contactless integrated circuits.
Although this tuning is generally performed at the time of commissioning the antenna circuit, the latter is then subjected to variations in humidity and temperature that can substantially affect its resonance frequency. The presence of metal objects in the proximity of the antenna circuit further influences the resonance frequency. The antenna circuit must therefore be tuned again on site to take its conditions of use into account. It must then be tuned at regular intervals if the reader RD is to have an optimal communication distance.
The tuning of the antenna circuit can be performed manually but this solution requires the intervention of qualified staff. Furthermore, a manual tuning can prove difficult in certain conditions of use. As an example, a contactless badge reader dedicated to controlling access in high altitude transport facilities (e.g., chair lifts, cable cars, etc.) will be considered. Due to the poor weather conditions and the very low temperatures, it is difficult to adjust the components of the antenna circuit bare-handed, using a screwdriver.
In this case, an electrically tunable antenna circuit is provided comprising embedded equipment that automatically tunes the antenna circuit (“autotuning”), once a day for example.
FIG. 2 represents an electrically tunable antenna circuit 20, comprising electrically adjustable capacitors C1, C2 for that purpose. The capacitor C1 comprises a capacitor C10 in parallel with a plurality of switchable capacitors C1i. The capacitor C2 comprises a capacitor C20 in parallel with a plurality of switchable capacitors C2j. Each capacitor C1i, C2j, respectively, is switchable by an electric relay RLi, RLj, respectively, arranged in series with the capacitor. The relays RLi are driven by signals Si and the relays RLj are driven by signals Sj. Thus, the combination of signals Si determines the value of the capacitor C1 and the combination of signals Sj determines the value of the capacitor C2.
FIG. 3 represents curves of phase φ and of impedance Z of the antenna circuit 20 when it receives an excitation signal Se oscillating at the frequency F0, such as 13.56 MHz for example.
The tuning of the antenna circuit 20 to the frequency F0 theoretically involves finding a pair of determined values of the impedance Z and of the phase φ, generally Z=50Ω and φ=0°.
However, a piece of equipment for measuring the phase Φ and the impedance Z is generally too costly and cumbersome to form an embedded piece of equipment.
Thus, as shown in FIG. 4, the embedded equipment performing the automatic tuning of the antenna circuit generally includes a simple voltage standing wave ratio (VSWR) meter TMR arranged between the antenna circuit 20 and the reader RD, and a tuning device TC1 supplying the signals Si, Sj for controlling the relays of the antenna circuit. The VSWR meter TMR measures the voltage standing wave ratio and the device TC1 tunes the antenna circuit so that the voltage standing wave ratio is minimal at the frequency F0.
This tuning method is simple and inexpensive, but the precision of it is mediocre. Indeed, the voltage standing wave ratio provides an indication which, although it is pertinent, can be considered in the light of experience acquired, to be insufficient in itself to achieve reliable and precise tuning. Furthermore, an adjustment of the antenna circuit based on the voltage standing wave ratio is long and difficult to find. Many combinations of the signals Si, Sj must be tested before finding the combination taken to be optimal.
Thus, it is desirable to provide a method for tuning an antenna circuit that is more precise than the classical method described above. It is also desirable to provide a tuning method that is precise and that can be implemented by embedded equipment that is simple and affordable to produce. It is also desirable to provide an antenna circuit that is simple to tune.